The present invention relates to a method for treating a gas to neutralize a SO3 component contained in e.g. a gas generated by combustion of a fuel containing a sulfur content by e.g. a boiler, inexpensively, efficiently, simply and safely without using a large scale installation.
When a fuel containing a sulfur content represented by heavy oil, coal or coke, is burned, or when a raw material containing sulfur, such as an iron ore, is burned, SO3 or H2SO4 will be contained in the exhaust gas, thus causing corrosion of an apparatus or air pollution. SO3 or H2SO4 will react with water vapor contained in the exhaust gas to form a sulfuric acid mist, which causes a white smoke, a purple smoke, a brown smoke or a black smoke (hereinafter referred to as a purple smoke, as a general term including a hardly extinguishable white smoke). The sulfuric acid mist which turns into such a purple smoke, will present a damage to a human body or an animal or plant at the falling site. Further, the ash dust deposited in a flue, etc., is likely to be discharged as acid smut containing sulfuric acid substantially due to e.g. a load change, thus leading to a problem of deteriorating the environment in the form of an acid dustfall.
Accordingly, in order to remove SO3, H2SO4 and a sulfuric acid mist (in this specification, these may generally be referred to as a SO3 component), a method has heretofore been employed wherein a slurry having e.g. an oxide or hydroxide of calcium or magnesium dispersed in an organic solvent, is preliminarily added to the fuel to prevent formation of the SO3 component, or wherein such a slurry is added to a gas after the combustion to neutralize the SO3 component. However, with such methods, the additives are likely to deposit on a heat exchanger in the boiler, and when a large amount of the additives deposit thereon, the operation of the boiler will be hindered, and accordingly, it is difficult to use the additives in a large amount.
Further, to actively neutralize SO3 in a flue, a method has been employed wherein a powder of e.g. calcium hydroxide, magnesium oxide or magnesium hydroxide or a slurry having such a powder dispersed in water, is injected to the flue after the exhaust gas has passed through an air preheater. However, in the case where the powder itself is injected by this method, as the fine powder poor in flowability is injected by means of e.g. a screw feeder, volumetric feeding will be poor, and no stable effect is likely to be obtainable. Further, such a powder is likely to agglomerate, and it is hardly uniformly dispersible and its effect as a neutralizing agent is low. Furthermore, in the case of injecting it in the form of a slurry, the powder contained in the slurry is likely to deposit on and clog the transportation line for injecting the slurry, whereby it is difficult to use it stably at a constant amount.
Further, in the case of using e.g. magnesium oxide, since magnesium oxide has a low reaction efficiency, it is necessary to add an excessive amount of a magnesium oxide powder. In such a case, unreacted magnesium oxide will remain in the flue, and the magnesium oxide may cause problems in dumping, since magnesium oxide has a low solubility in water. Further, it is difficult to keep the amount of injection to be constant, since the powder is fine and has a poor flowability.
On the other hand, a method of injecting ammonia to the flue, may be mentioned. However, there are restrictions relating to handling of e.g. a high-pressure gas and problems relating to the storage temperature, and further, an additional large scale installation is required. Further, when the SO3 component is removed by ammonia, if an adequate injection amount is not maintained, an ammonium hydrogen sulfate will form. If the ammonium hydrogen sulfate is deposited on the apparatus, it will cause a trouble. Accordingly, ammonia is required to be injected excessively, and if the excessive ammonia is released in the atmospheric air, such will be problematic from the viewpoint of the environment safety.
Further, it is required to remove a SO3 component also in the treatment of an exhaust gas of combustion of a waste liquid, a waste oil, a waste gas or a solid waste in e.g. steel manufacturing, iron manufacturing, nonferrous metal refining, glass melting, manufacturing of sulfuric acid or manufacturing surfactant, and an efficient and safe neutralizing treatment method for the SO3 component, is desired.
It is an object of the present invention to provide a method for removing a SO3 component from a gas containing the SO3 component, efficiently, simply and safely.
The present invention provides a method for treating a gas containing a SO3 component, which comprises adding a sodium carbonate powder having a mean particle diameter of at most 20 xcexcm to the gas to remove the SO3 component from the gas.